1. Field of the Invention
The present invention relates to wafer metrology, and more particularly to obtaining overlay measurements for a semiconductor wafer using zero-order cross polarization measurements obtained from a periodic grating formed on the wafer.
2. Related Art
Semiconductor devices/circuits are formed on semiconductor wafers by depositing and patterning layers of materials. In general, the features of the devices/circuits are formed onto the layers of deposited materials using a patterning process.
In a typical patterning process, the features of the devices/circuits are laid out, one layer at a time, on a series of photomasks (masks). The layout of the features of the devices/circuits on the masks are transferred, one mask at a time, onto the deposited layers of materials. Misalignment of these masks, which is generally referred to as “overlay error”, can adversely affect the performance of the devices/circuits.
To reduce overlay error, alignment marks, such as box-in-box or frame-in-frame overlay marks, are typically patterned onto the wafer and on layers deposited onto the wafer. At present, optical imaging systems are widely used to detect these alignment marks. However, a conventional optical imaging system typically has an accuracy of only about 5 to 10 nm. The continual shrinkage in the feature sizes of devices/circuits will likely require greater accuracy.